Electromagnetic recording head



Feb. 12, 1952 M. F. ROYSTON 2,585,040

ELECTROMAGNETIC RECORDING HEAD F iled March 23, 1949 I INVENTOR MARVIN EROYSTON BY @40 W M z/IMW ATTORNEYS i atentecl Feb. 12, 1952 ELECTROMAGNETIC RECORDING HEAD Marvin F. *Royston, -Poughkeeps'ie N. sY.,.assig1 ior to Frederick Hart & Co., Inc., 'Boughkeepsie,

N.'iY.,=a. corporation of Delaware Application Mai-ch23, 1949, Serial Nm 83.093 7 Claims. (01. 179-10031) This invention :relates :to electromagnetic recording heads "for: the trecordingeof sound-tomrecords of suitablexmaterial. While of generalapplicability to the field of soundrrecording, .the invention isparticularly applicable :to the embossingtof veryfine laterally-modulatedsoundegrooves infilm.

Primaryobjects 'of'zthe inventionare the provision :ofaneflicient recordingzheadiof very simple construction which provideshighzfidelity :recording'and yet is relatively inexpensive.

The recording ahead of the present invention is of v: the variable reluctance type employing-two air gaps in the'magnetic circuit on :opposite sides of the'pivo't point of 'thearmature. This system is often termed 'a double-ended armature :or bal ancedxarmature system. In such recordingheads the cross-sectional area of the armature'and the pole .piecesmust be large enough to provide an eificient magnetic :path. 'Also i a centering spring system must be provided which is stiff enough to prevent collapse of the armature against a pole due to D.-C. ifiux :produced 'by the permanent magnet. The mass ormoment of inertia of the armature, together with the centering spring system, results .in .a moving mechanical system having a natural period of vibration. Inorder to .obtainlhigh fidelity response from such a 'recording head, it:is necessaryto makethi's natural period :of vibrationas' high as possible, since the frequency response extends only to a limited extent beyond'the' peak corresponding to the natural period of vibration, and drops ofi very rapidly beyond this limited :range. p

In accordance with the -present invention a single coil is utilized-and a novel pivot and spring suspension is'provided which pivots Lthe-armature midway alongthe axis of thecoil without substantially reducing the effective-cross-sectional area of the armature and "the pole pieces. This novel suspension permits the distance from the pivot point to the air gaps to be reduced to a minimum, thereby reducing ithe eiiective mass and moment of inertia of the armature. Thisreduction in eifective mass and moment of inertia is accomplished without'reducing:theefiiciency of the magnetic circuit. As aresult, :the resonant peak of the armature and suspension is high in the audio range and producesa very high fidelity recordinghead.

It is also important that the pivot axis of the armature be very rigid so as to prevent any lateral motion of the armature which wouldamount to lost-motionandreducetheefiiciency of the head. It is also importantthatthesuspension be simple 12' and rugged, and-capable .of.manufacture with minimum variation indimensionsand tempering.

In accordance with .theinvention a. knife edge is 1 provided .midwayrbetween the air gaps, and r in the center of the.coi1,. so es to provideafixedrigfi pivot axis .ior the armature. This v-knife edge allows oscillation of. thearm-ature: soas .to produce lateral movement of the :associated .stylus, but prevents any :lateralmovement. of the armature as a whole. vOn .thewoppositesideof the armature a flat spring .isprovided whichbearsagainst the armature-at two points eveniyspaced .on .opposite sides.of thekniie-edge-lt provide equal and opposite torques. Thisispring provided with legs whichareheldinthe polepiece onithat side of :the armature with provision .foradjus'tingthe spring tension .and for .rigidly securing it .its adjusted osition.

The invention will .he.more-fullyunderstood by reference to the following :detailed description thereof taken .in conjunction with the drawings in which:

Fig. lis aver-tical crossesectiomtakenalong the line 1-4 of Fig-=2; I

Fig. .2 isalateral .cross section taken along the line 2-.2- of Fig. 3;

1Fig.s3 ;a..horizontalicross-section taken. along.

the line 3-- -3; ofl 'igm;

Fig. Ads 2. bottom viewof itheirecording head; -Fig.: 5 is a ,plan view thereof :Fig. .6 adetail showing the .knife edge forming the horizontal pivot axis; and Fig. 7 is a detail showing ithespringimember.

' Referring to the drawings, .a top sinzzporting plate 10 .of nonsmagnetic material has :afiixed thereto, bymeans-.-of'mounting.screws M a painof channel pole-pieces 13 and M. The cross-sections of these pole pieces are U-shaped. The sides of the polepieees are substantially horizontal and the \fronhportions 4 5 and 1.6 thereof pro-- ject inwardlyzin horizontal alignment to .define a pair=of1-air gaps L1 and i It. TheseaingaPsJ Land is are; separated .in a :vertical direction and are relatively.longrinztheaxialiiirection and relatively 'narrow. in the lateral \direction. A apermanent magnet 19 depositional in abutment with .the rear endsaof thepole gpieces land =14 with the magnetic 1 poles of the magnet adjacent respective -poleqaieces. .Thezdirection of magnetization islindicatedibyearrow 20. lnseonsequencera constant :Dr-C. magnetic 1 field I-is ,produced=across -air aps H and 1; l .8-zand=extends::in .thesame: direction across \both sgaps.

An: armature 21 :01 magneticsmateriahsucmas soft ironiszpositioned withinathe air gapszand has ries the recording stylu's 23. c

The cross-sectional areas of the armature and the edges of the pole pieces forming the air gaps are sufficiently large to form an eificient ma netic circuit.

The armature is held in position by a suspen sion comprising knife edge 24 on plate 25, and cooperating abutments 26 on spring '21. Plate 25 is secured in a suitable recess in the front portion of pole piece M with the knife edge. midway between the upper and lower projecting portions of the pole piece. The knife edge is accordingly midway between the air" gaps l1 and I8. The knife edge extends in-'theaxial direction and thus allows oscillation of the armature to produce lateral movement of'the stylus 23. However, the knife edge is rigidly positioned in the pole piece and provides a rigid pivot-axis for the armature. Hence lateral movement of the armature as a whole 'is prevented, thereby preventing lost motion which would impair the response of the head. Although the knife edge 24is advantageously broken into'two spaced segments as shown, it could be madecontinuous if'desired. I

' The cooperating spring 21 is bent in the form of a U witha relatively flat spring face 28. Abutments 26, advantageously in' the form of short knife edges as shown; project from the face 28 so as toabut the armature at spaced points. Spring 21 may be conveniently formed by flattening the central portion of a round wire. forming the knife edges 26 during the flattening operation; and bending the ends of the wire to form legs 29. The spring may thereafter be hardened and tempered. Thelegs 29' of the spring fit into mounting holes in the upper and lower sides of pole piece l3; and centrally located with respect to the corresponding air gaps. When in position, the abutments 26'are'parallel to knife "edge 24 but are spaced apart in a vertical direction, that is, laterally of knife edge 24', by equal amounts on each side of the lattert-Abutments 26-hence apply equal and opposite torques'to armature'Zl about knife edge -24. Spring 21 is' a'dvantageously located sothat the center 'line of abutments 26 is perpendicular to-the knife edge 24 at apoint midway of the latter. Thus the-armature is pressed securelyagainst' knife edge "24' and any oscillation or vibration about a vertical axis is prevented. a

, Adjusting screws 3| bear against the ends of legs 29 and press the spring into engagement with the armature. In the initial assembly th -force of the spring'on the armature is made suflicient to center the armature in the air gaps and insure that the armature does not collapse against a pole piece due to the -D.-C. fiux produced by the permanent'magnet l9. 1 When the proper tension is obtained, screws 32 are tightened against upper and lower legs29 to clamp the spring securely in position. It has been found advantageous to locate screws 32 as close as possible. tothe inner edges of the'upper and-lower-sides of Dole piece [3, that is,- close to-the air gaps, so that spring. member 21 is rigidly secured as'close as possible tothe bends 35.= In'this way undesired vibrations and spurious response peaks-ma'y be avoided.

Suitable V-shaped indentations may be made in armature 2| to cooperate with knife edge 24 and abutments 26.

An actuating coil 33 is positioned within the space between the pole pieces and encircling the armature 2!. When energized with sound frequencies through lead 34, the armature is vibrated according to the variable reluctance principleand causes the stylus 23 to'move laterally in accordance with the sound frequencies.

-It will be apparent that the armature suspension described above serves to provide a pivot axis which is midway between the air gaps and in the center of coil 33-, and that the axis of rotation is rigidly established to allow oscillation of the armature about the pivot axis but no movement in any other direction. This is accomplished without increasing the vertical separation of the air gaps over that required to accommodate coil 33. Furthermore, the manner of suspension in no way decreases the effective cross-section of the armature. As a result, the height of the armature is kept to a minimum with consequent low mass and moment of inertia, without decreasing the efficiency of the magnetic circuit. This permits placing the resonant response peak -high in the audio range, and therefore enables a high fidelity recording headto be produced.

As an example for purposes of'illustration only, the response curve for one recording head made in accordance with the invention showed efficient operation from 20 to 11,000 cycles per second, with the resonant peak at about 9,000 cycles per second. When used for embossing sound grooves in a film of cellulose acetate or other plastic, the damping provided by the recording mediumsufficed to damp out this resonant peak and yield a fiat response.

Under some conditions of operation the recording medium may not provide sufficient damping to flatten out the resonant peak. This might occur in cutting grooves in lacquer or wax. In such event damping material may .be incorporated in the recording head to provide damping of the armature.

It will also be appreciated that the recording head of the invention is very simple and is composed of simple parts which can be readily manu- 1 An electromagnetic sound recording head which comprises a pair of U-shaped pole pieces positioned to define a pair of air gaps therebetween, magnet means for producing a magnetic field across said air gaps, an armature positioned within said air gaps and adapted to carry a stylus. a knife edge positioned midway between said air gaps and engaging one side of said armature to provide a pivot axis therefor, a unitary compres sional spring member having a-pair of abutments engaging the oppositeside of said armature at respective points spaced apart laterally of saidknife edge and on opposite sides thereof to -provide-substantially equal and opposite torques onaaaaoao said armature, and :an actuating :coil positioned in ithe-space .defined \by-said pole pieces and encircling said armature. p a

2. An electromagnetic sound recording head which comprises a pair of U-shaped pole pieces disposed with the open ends facing and in substantial alignment to define a pair of "air gaps, magnet-means for-producinga=substantial1y constant magnetic field in the same direction across said pair "of air gaps, a flat armature positioned within said air gaps andadapted to'carrya stylus, a knife edgeuposiitoned'midway between-said air gaps and engaging one side'of said armature to provide a pivot axis therefor'allowing oscillation of said armature "in said gaps, a unitary compressional spring member having a pair of K abutments between said air gaps engaging the opposite side of said armature at respective points spaced apart in a direction lateral of said knife edge and on opposite sides thereof to provide substantially equal and opposite torques on said armature, and an actuating coil positioned in the space defined by said pole pieces and encircling said armature.

3. An electromagnetic sound recording head which comprises a pair of channel pole pieces positioned with the openings thereof facing and the sides thereof in substantial alignment to de- 7 fine a pair of elongated air gaps, magnet means for producing a substantially constant magnetic field in the same direction across said pair of air gaps, a fiat armature positioned within said air gaps and adapted to carry a stylus, a knife edge positioned midway between said air gaps and parallel thereto and engaging one side of said armature to provide a pivot axis allowing oscillation of the armature in the air gaps, an elongated unitary spring member positioned on the opposite side of said armature and extending laterally of said knife edge, said spring member having a pair of abutments spaced therealong between said air gaps and bearing against said armature at respective points on opposite sides of said knife-edge, and an actuating coil positioned in the space defined by said pole pieces and encircling said armature.

4. An electromagnetic sound recording head which comprises a pair of channel pole pieces positioned with the openings thereof facing and the sides thereof in substantial alignment to define a pair of elongated air gaps, magnet means for producing a substantially constant magnetic field in the same direction across said pair of air gaps, a fiat armature positioned within said air gaps and adapted to carry a stylus, a knife edge positioned midway between said air gaps and parallel thereto and engaging one side of said armature to provide a pivot axis allowing oscillation of the armature in the air gaps and resultant lateral movement of the stylus, a unitary spring member extending between the sides of the pole piece on the opposite side of said armature and lateral of said knife edge, said spring member having a pair of abutments between said air gaps spaced apart laterally of said knife edge and bearing against said armature at respective points substantially equally spaced on opposite sides of the knife edge, and an actuating coil positioned in the space defined by said pole pieces and encircling said armature.

5. An electromagnetic sound recording head which comprises a pair of channel pole pieces positioned with theopenings thereof facing and the sides thereof in substantial alignment to define 6. a, pair :of elongated airl'gaps, magnet means for producing a substantially constant-magnetic-field inithe same-direction across said pair :of air gaps, a flat armature positioned within said air "gaps and adapted to :carry .a stylus, a plate mounted between the sides :of one ofsaid pole piecesrhaving a knife "edge midway between said :air gaps and parallel thereto, said knife edge engaging one side of saidcarmature.tovprovidea fixed/pivot axis allowing oscillation of the armature insaid air gaps,la U-shapedunitaryzspring member-have. ing :the .legs thereof mounted in :respective sides of the other .of said pole pieces with the face thereof extending along the other sideof said armature substantially perpendicular tosaid pivot axis, said springmember having a pairof spaced knifeedges parallel to the first mentioned :iknife edgeland bearing. against said armatureatiree spectiveipoints between said air gaps which are equally spaced on opposite sides of said pivot axis along a line perpendicular thereto, and an actuating coil positioned in the space defined by said pole pieces and encircling the armature, the plate bearing the knife edge and the face portion of the spring.

6. An electromagnetic sound recording head which comprises a pair of channel pole pieces positioned with the openings thereof facing and the sides thereof in substantial alignment to define a pair of elongated air gaps, a permanent magnet arranged to create magnetic fields extending in the same direction across said air gaps, a flat armature positioned within said air gaps and having a downwardly extending portion bearing a stylus, a knife edge positioned midway between said air gaps and parallel thereto and engaging one side of said armature to provide a fixed pivot axis allowing oscillation of the armature in the air gaps and resultant lateral movement of the stylus, a U-shaped unitary spring member having the legs thereof mounted in laterally extending holes in respective sides of the other of said pole pieces, the face of said spring member being elongated and substantially fiat and extending along the other side of said armature prependicular to said pivot axis and midway therealong, said spring member having a pair of spaced knife edges parallel to the first-mentioned knife edge and bearing against said armature at respective points between said air gaps which are equally spaced on opposite sides of said pivot axis along a line perpendicular thereto, and an actuating coil positioned in the space defined by said pole pieces and encircling said armature.

7. An electromagnetic sound recording head which comprises a pair of channel pole pieces positione with the openings thereof facing and the sides thereof in substantially horizontal alignment, the front portions of the sides of said pole pieces projecting inwardly to define a pair of vertically-separated axially-elongated air gaps, a permanent magnet positioned with the magnetic poles thereof adjacent respective rear ends of said 7 holes in respective projecting portions of the other of said pole pieces, the face of said spring member 'being elongated and substantially flat and extending along the other side of said armature perpendicular to said pivot axis and midway 5 therealong, said spring member having a, pair of spaced knife edges parallel to the first-mentioned knife edge and bearing against said armature at respective points between said gaps which are equally spaced on opposite sides of said pivot axis along a line perpendicular thereto, adjusting means engaging the legs of said spring member to adjust the tension thereof against the armature, clamping means engaging said legs near the face of said spring, and an actuating coil positioned in the space defined by said pole pieces and encircling the armature, the plate bearing the knife edge and the face portion of the spring. MARVIN F. ROYSTON.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 957,403 Baldwin May 10, 1910 1,663,885 Holland Mar. 27, 1928 1,709,571 Harrison Apr. 16, 1929 1,717,263 Rice June 11, 1929 1,757,703 Curtis May 6, 1930 1,980,137 Hubner Nov. 6, 1934 2,141,820 Paredes Dec. 27, 1938 2,241,107 Woodruff May 6, 1941 2,286,178 Kornei June 9, 1942 2,412,718 Dally Dec. 17, 1946 

